1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine, a facsimile and a printer. More specifically, it relates to an image forming apparatus comprising a rotatable developing device for developing a plurality of colors, and a transferring means as an intermediate transfer member for forming toner images of a plurality of colors on a photosensitive member and transferring the toner images on the photosensitive member or a rotatable transfer drum supporting a recording medium for transferring the toner images on the photosensitive member to the recording medium.
2. Related Background Art
Conventionally, as the configuration of an apparatus for forming a multiple color image according to an electrophotographic system, a multiple color image is obtained by forming a latent image by the selective exposure on an image bearing member (hereinafter it is referred to as the “photosensitive drum”) as a drum shaped electrophotographic photosensitive member charged uniformly by a charging device, disposing a plurality of developing devices storing developing agents (hereinafter it is referred to as the “toners”) of different colors on a rotary as a rotation selecting mechanism, developing with the developing device storing a predetermined color faced with respect to the above-mentioned photosensitive drum, transferring the toner image to a recording medium, and furthermore, executing the developing and transferring operations to each color.
There is a method provided with an intermediate transfer member as the recording medium in addition to a paper and an OHP sheet, for temporarily collecting the colors as an intermediate role before the transfer onto the paper or the OHP sheet. A method using an intermediate transfer member comprising for example a resin sheet made of a PET, or the like for the transfer by each color from the photosensitive drum and then transfer onto the paper or the OHP as the recording medium has been proposed.
In the case the positioning accuracy of the colors is low, specifically, in the case displacement of 50% or more with respect to the sub scanning pitch is generated, the unevenness or the color displacement can be observed visibly so that image failure called banding or jitter is generated in many cases. As the cause of the sub scanning displacement, the following two can be presented.    (1) The exposing position (sub scanning position) on the photosensitive drum is displaced due to the mechanical accuracy error, the control error, the speed fluctuation, the internal vibration, and the external vibration.    (2) The transfer position onto the recording medium is displaced due to the mechanical accuracy error, the control error, the speed fluctuation, the internal vibration, and the external vibration.
In order to improve the image failures, the following proposals have been provided as the conventional inventions.
The laser beam displacement in the sub scanning direction is detected by an photo electric conversion element, and the laser exposing position is adjusted in the sub scanning direction by a galvano mirror disposed on the front side of a polygon mirror (see the Patent Article 1).
The polarizing mirror for an exposing beam is corrected constantly by detecting the dynamic pitch fluctuation in the sub scanning direction by measuring the speed fluctuation of the photosensitive drum by an encoder, or the like. Moreover, the frequency of a writing clock is corrected according to the correction amount (see the Patent Article 2).
[Patent Article 1] The official gazette of the Japanese Patent Application Laid Open (JP-A) No. 2001-253115
[Patent Article 2] The official gazette of the Japanese Patent Application Laid Open (JP-A) No. 10-197810
However, as shown in the conceptual diagrams of FIGS. 11 and 12, according to the conventional method, in a process for producing an electrostatic latent image on a photosensitive drum by an optical means such as a laser beam, at the time a developing rotary having a large rigidity starts or stops moving (see B1 of FIG. 11), the units of the copying machine main body may be vibrated. Particularly in the case the vibration is transmitted to the laser unit (see B2 of FIG. 11), the scanning interval in the sub scanning direction is displaced so that stripes called banding or jitter are generated (see B3 of FIG. 11).
Then, according to the above-mentioned conventional examples, for example, methods of preliminarily detecting the laser unit vibration by a detecting means such as a vibration sensor for vibrating the laser beam in the opposite phase based on the data, controlling the angle of the mirror in the optical system, controlling the rotation speed of the photosensitive drum, or the like have been proposed. However, due to the need of a special mechanism such as the detecting means and the correcting means, the apparatus cost is increased.